As defined, a snatch block is a rotatable sheave or pulley having supporting sideplates. At least one of the sideplates may be moved outwardly to expose the sheave and the line carried thereon. Thus, the block can be opened quickly to receive a line without the necessity of feeding the bitter end of the line into the block and pulling it through.
Especially in sailing vessels, snatch blocks are often employed because of their portability and versatility. The block usually carries a shapshackle, which allows the block to be quickly and easily moved from one support location to another, such as a padeye, toe rail, or traveler track fitting.
Snatch blocks used on sailing vessels have all employed a similar design. The sideplates extend away from the block to a head and a fitting such as a snap shackle. A tongue extends sideways from the head and is engaged by a hasp which is hinged to one of the sideplates. One problem with this design is the difficulty of opening the hasp, especially if there are even moderate loads on the block. Another potential problem is the failure to return the hasp to a fully closed position, causing a failure when the block is loaded.